The ranks of "S.H.I.E.L.D." keep on growing. Just a few days after adding Luke Mitchell to the regular cast of "Agents of S.H.I.E.L.D.'s" third season, TV Line has learned that ABC's Marvel series has also upped Henry Simmons to a series regular.

Simmons, who plays S.H.I.E.L.D. engineer Alphonso "Mack" Mackenzie, made his debut in the season two premiere and was most often paired with other new additions Lance Hunter (Nick Blood) and Bobbi "Mockingbird" Morse (Adrianne Palicki). Mack also struck up a friendship with Agent Leo Fitz (Iain De Caestecker), who was struggling with his communication skills following a near-death experience. Despite not being a series regular, Simmons appeared in every episode of season two, even as the show appeared to be willing to write his character out; he became temporarily possessed by a rage-filled alien entity in the midseason finale and abandoned Agent Coulson's mission out of frustration towards the end of season two. The finale, however, found him working alongside the rest of the core S.H.I.E.L.D. crew once again.

"Agents of S.H.I.E.L.D.'s" cast continues to grow as it preps for it's junior year. No departures have yet to be announced, meaning that the show retains all six of it's initial leads -- Clark Gregg, Ming-Na Wen, Brett Dalton, Chloe Bennet ,Iain De Caestecker and Elizabeth Henstridge -- as well as the pair of actors (Blood and Palicki) added in season two. In addition to more of Mack, the electricity-powered Inhuman Lincoln (Luke Mitchell) will also be a series regular when season three debuts on ABC this fall.